Help with Custom Fields

I am running another script which connects to twitter on my site. I am trying to use a custom field to insert the FRIENDS_USERNAME.

My custom field is called “twittername” I usually use the following code to include custom fields in HTML

$key="twittername"; echo get_post_meta($post->ID, $key, true);

But I don’t know how to include in php. Can someone please take a glance at my code below and see where I might be going wrong?

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<?php
     //username and password to pass to twitter -
     $username = 'xxxxx';  //these dont have to be in caps
     $password = 'xxxxx';

     //$twitterUser = 'YOUR_FRIENDS_USERNAME';
     $twitterUser = '$key="twittername"; echo get_post_meta($post->ID, $key, true);';
?>
<?php endwhile; else: ?>
<?php endif; ?>

I’d appreciate any help! I am not good with php!

Thanks,

Jon